I'm guessing you're getting about a 10:1 gear ratio since the drive gear on your motor looks to have +/- 11 teeth and I'd estimate that if the gear on the scope was a completely circle, it would have ~100+ teeth. In my case, I have a 6.5:1 ratio (20 teeth on the drive gear and 130 teeth on the scope side). And now I'm adding a stepper with a 5.13:1 gearbox.

I did some additional experiments with the geared stepper and discovered that the problems I was seeing with inconsistent (even backwards) movement went away when I increased the step delay in the WMH driver. I had been using a 40us delay per step, and am now using 100us. I'm guessing it was trying to micro step too quickly. I bet that's the issue I was getting on my regular stepper as well. I'll have to see if it performs better with the higher step delay. I also have the option of turning micro stepping off completely. The only reason it was used in the first place was to smooth out the motion and not to actually increase resolution. But if the stepper loses torque in the process, it might be better to just use the native step size of the motor. The gearing should improve both torque and resolution.

I had been looking for your original CAD files to adapt your gear design to my layout. However I can do that pretty readily in Fusion360 if I need to. I also probably have a Nema bracket around somewhere if I end up trying out your design. I'm still hopeful that I can get the belt driven solution to work.

Ha... our messages crossed. I think the timing is indeed my problem. With the geared stepper, the inconsistent (even reversed) movement happens under zero load. I think I'm just pulsing the motor too quickly.

Hah! Looks like esb was all over it! We both posted essentially the same thing!

Correct, the ratio in that case is 13:1, the full circle of gears on the focus ring is 130 gears, if I remember correctly.
But you can take A LOT of strain off the motor when moving out, using the rubber band support see in my earlier pictures. That can reduce the strain on the motor by an additional 50% or more, depending on how strong the rubber bands are you choose to install.

Anyway, there is a lot you can do mechanically to support a weak motor with a poor transmission ratio.

And don't forget the reduced backlash. That is something that is difficult to eliminate using the belt drive.

I also added the FreeCAD files to the stl folder. Not sure you can translate them into Fusion360 format, but if you can, there they are!

Best

Jo

And a problem like this wouldn't be complete without a facepalm. I just noticed with the stepper attached to my RedCat, if I ran it from 0 (fully retracted) to nearly max (500) and then back to 0, it kept missing the zero point by a few degrees (it was always short). It turns out the grub/set screw for the pulley attached to the stepper is loose. Double-d'oh! That means I have at least 10+ steps of backlash. I'm guessing it will work much better with a tight pulley.

Well, all problems in astrophotography are mechanical or optical in the end....

Consider yourself getting off easy! When I did the Rowan belt mod for my Orion Atlas mount, one of the set screws on a pully deep inside the mount wasn't tight enough. Took me a month to figure out why I was getting such bad backlash in Dec movements! So I had to tear the mount apart to tighten that screw. I put blue Loc-Tite on those screws, just because I was mad at them!

With my existing stepper, backlash now seems to be around 1 step. I can see clear consistent movement for even a single step and when changing directions at most that first step doesn't show full movement. I can see that with large moves in one direction followed by single step moves in the other. When does darkness get here? I really can't way to try it out again.

Its especially ironic because the grub screw is one of the first things I advise others to check for many different kinds of 3d printer issues. I need to take my own advice.

I still want to go head-to-head with your design.

Identical optical train, so it's just the focuser.

We got to have a competition, Eric!!!



Best, Jo

I win, been there done that I hate Helical's

That is cool! The key is the tension pulley. That makes all the difference for eliminating backlash, I bet.
Alright, you can be part of the Olympiad here.
I am not chickening out. I am still confident I will win.

Jo